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67 lines
2.8 KiB
67 lines
2.8 KiB
1 year ago
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# Snapshot File format
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This docoment describes the binary file format of the boa snapshot files.
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## Header
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The header composes the first part of the snapshot.
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| Field | Description |
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| --------------------- | ------------------------------------------------------------------------------------------------------------ |
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| signature `: [u8; 4]` | This is used to quickly check if this file is a snapshot file (`.boa`) |
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| guid | Guid generated in compile time and backed into the binary, that is used to check if snapshot is compatibile. |
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| checksum | Checksum that is used to check that the snapshot is not corrupted. |
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## JsString Table
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After the `Header` the table containing `JsString`s each entry contains
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| static? `: u8` | length: `: usize` | `JsString` elements `: [u16]` |
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| -------------- | ----------------- | ----------------------------- |
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| 0 | 5 | `'H', 'e', 'l', 'l', 'o'` |
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| 1 | - | 3 |
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| ... | ... | ... |
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If it's a static string then it's elements comprise the index into the `STATIC_STRING`s.
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## JsSymbol Table
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| `JsSymbol` hash `: u64` | Description (index into `JsString` table) `: usize` |
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| ----------------------- | --------------------------------------------------- |
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| 200 | 0 |
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| ... | ... |
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## JsBigInt Table
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| Length in bytes `: u64` | Content |
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| ----------------------- | ------- |
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| 32 | ... |
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## Shapes (Hidden classes) Table
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### Unique Shapes
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| `[[prototype]]` `: u32` (index into `JsObject` table) | property count `: u32` | key-value pairs |
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| ----------------------------------------------------- | ---------------------- | --------------- |
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| | 0 | |
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| | ... | |
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### Shared Shapes
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| previous `: u32` | flags | transition |
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| ---------------- | ----- | ---------- |
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| `MAX` (root) | ... | `x` |
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## JsObject Table
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| length | Content |
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| ------ | ------- |
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| 200 | ... |
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| ... | ... |
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## JsValue Encoding
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type `: u8` (JsValue discriminant, Boolean, Null, etc) followed by the value if it exits.
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If following the `JsValue` is an `JsString`, `JsSymbol`, `JsBigInt`, `JsObject` then the
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following value will be an index into the appropriate tables.
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